The present invention relates to a control system for a continuously variable transmission for a motor vehicle.
The above-listed U.S. Pat. No. 4,735,113 (Yamamuro et al.) discloses a creep torque control and a lock-up control for a continuously variable transmission. According to this known control system, a selected one of a lock-up control valve and a throttle valve is subject to a hydraulic fluid pressure signal generated by an electromagnetically operated valve. The lock-up control valve has a lock-up position and a lock-up release position and assumes one of the two positions under the control of the electromagnetically operated valve. The throttle valve generates a throttle pressure which is supplied to a forward clutch for a forward drive or a reverse brake for a reverse drive. The magnitude of the throttle pressure is adjustable by the electromagnetically operated valve. The electromagnetically operated valve is connected to the lock-up control valve or the throttle valve, selectively, under the control of a shift command valve. The shift command valve has a spool longitudinally movable by a shift motor. The spool is movable within a normal stroke range between a minimum reduction ratio position and a maximum reduction ratio position and also movable beyond the maximum reduction ratio position into an overstroke range next adjacent to the normal stroke range. The spool is formed with two axially spaced lands which function to connect the electromagnetically operated valve to the lock-up control valve to subject the lock-up control valve to the hydraulic pressure signal during movement of the spool within the normal stroke range and disconnect the electromagnetically operated valve from the lock-up control valve during movement of the spool within the overstroke range. During movement of the spool within the normal stroke range, the lands on the spool function to disconnect the electromagnetically operated valve from the throttle valve, while during movement of the spool within the overstroke range, the lands on the spool function to connect the electromagnetically operated valve to the throttle valve. A constant pressure regulator valve generates a constant hydraulic pressure. This constant hydraulic pressure is allowed to act via a signal pressure port on the lock-up control valve when the electromagnetically operated valve is disconnected from the lock-up control valve during the movement of the spool within the overstroke range to keep the lock-up control valve at the lock-up release position, causing a lock-up mechanism of a hydrokinetic torque transmitting unit to assume a lock-up release state. Upon or immediately after a driver's demand for moving the motor vehicle from a standstill, the spool of the shift command valve moves from the overstroke range to the maximum reduction ratio position of the normal stroke range to connect the electromagnetically operated valve to the lock-up control valve. Subsequently, the lock-up control value is allowed to shift between the lock-up release position and the lock-up position under the control of the electromagnetically operated valve. The electromagnetically operated valve holds the lock-up control valve in the lock-up release position until the vehicle speed exceeds a lock-up vehicle speed value, and subsequently shifts the lock-up control valve to the lock-up position when the vehicle speed exceeds the lock-up vehicle speed value.
Let us now consider the case where the electromagnetically operated valve fails to operate normally and sticks to a position where the signal pressure port of the lock-up control valve is drained when the electromagnetically operated valve is connected to the lock-up ,control valve. In this case, the lock-up control valve is shifted to the lock-up position immediately after the spool of the shift command valve moves from the overstroke range to the maximum reduction ratio position of the normal stroke range. This means that the lock-up mechanism assumes the lock-up state upon moving the motor vehicle from a standstill. This results in an engine stall and the motor vehicle fails to start moving from a standstill.
An object of the present invention is to improve the control system mentioned above such that the motor vehicle can start moving from a standstill even if the electromagnetically operated valve should fail to operate normally.